A similar short description can be found in [1]} designed for metal clusters. The main idea is to start, analogous with the stationary version (TF), from the quantum mechanical action and the variational principle of least action:
\(\mathcal {A}(t)=\int \limits _{0}^{t}\langle \Psi (\tau )|i\hbar \partial _{\tau }-\hat{H}|\Psi (\tau )\rangle d\tau \) 
